High molecular weight RTPU's are single- or two-phase polymers that can be prepared by the reaction of approximately stoichiometric amounts of a low molecular weight diol chain extender (molecular weight of not more than 300) and optionally a high molecular weight diol (molecular weight generally in the range of from about 500 to about 8000) with a diisocyanate. These RTPU's have a glass transition temperature (T.sub.g) of not less than 50.degree. C. and typically have a hard segment content of not less than 75 percent. The disclosure and preparation of RTPU's is described, for example, by Goldwasser et al. in U.S. Pat. No. 4,376,834, incorporated herein by reference.
Because RTPU's tend to have a weight average molecular weight greater than 200,000 and a capacity for intermolecular hydrogen bonding, these polymers require very high thermal and/or mechanical energy input to generate the homogeneous, plasticated melt needed for thermoplastic forming processes such as injection molding or extrusion. The requirement for high energy input in the melting/plasticating stages may stall the screw of an injection molding machine or extruder, or may result in localized overheating of material due to high shear in the transition zone of the screw, which in turn causes polymer degradation and splay defects in the molded part.
One method for overcoming these processing problems is to incorporate a chain stopper such as a monofunctional alcohol into the formulation, as disclosed by Quiring in U.S. Pat. No. 4,371,684, incorporated herein by reference. Unfortunately, physical properties of the RTPU product, such as T.sub.g and toughness, suffer at the expense of improved processability. Alternatively, urethane molecular weight can be reduced by deliberately polymerizing with a deficiency of isocyanate (that is, an excess of hydroxyl groups). As disclosed by Ulrich in "Kirk-Othmer: Encyclopedia of Chemical Technology", Vol. 23, 3rd Ed., p. 598 (1983), such products are preferred for use in extrusion processes. However, in practice, the molecular weight of the polymer may be very difficult to control for some large scale manufacturing processes using this off-ratio approach. It is therefore desirable to find a practical way to improve processability of RTPU's without sacrificing physical properties.